The present invention relates to a process for preparing .alpha.-methylstyrene/acrylonitrile copolymers, and more particularly to a process for preparing a copolymer having excellent transparency, heat resistance and strength in a high conversion in a short time by copolymerizing .alpha.-methylstyrene and acrylonitrile with at least one member selected from the group consisting of styrene, chlorostyrene, para-methylstyrene, t-butylstyrene, acrylic ester and methacrylic ester in the presence of a specific peroxide by suspension or bulk polymerization.
When .alpha.-methylstyrene, acrylonitrile and at least one compound capable of copolymerizing with .alpha.-methylstyrene and acrylonitrile selected from the group consisting of styrene and a substituted styrene such as chlorostyrene, para-methylstyrene or t-butylstyrene; an acrylic ester such as methyl acrylate, ethyl acrylate or butyl acrylate; and a methacrylic ester such as methyl methacrylate, ethyl methacrylate or butylmethacrylate are copolymerized, it is necessary to employ .alpha.-methylstyrene in an amount of at least 10% by weight (hereinafter referred to as "%"), preferably at least 20%, of the whole monomers to be used to produce a copolymer containing at least 10%, preferably at least 20%, of the .alpha.-methylstyrene units, in order to obtain a copolymer which has heat resistance sufficient for practical use.
When it is desired to prepare a copolymer containing a large amount of .alpha.-methylstyrene by suspension or bulk polymerization to obtain a copolymer having an excellent heat resistance, a process employing as a polymerization initiator an organic peroxide such as t-butyl peroxybenzoate, t-butyl peroxyacetate or di-t-butyl peroxide is adoptable. However, in a process employing such initiators, it is necessary to employ a large amount of the initiators regardless of the polymerization temperature. Consequently, an .alpha.-methylstyrene/acrylonitrile copolymer prepared by such a process is very low in the degree of polymerization and is poor in utility as a molding material. On the other hand, in case of using these initiators in a decreased amount for the purpose of raising the degree of polymerization, so-called dead end polymerization occurs. Thus a high conversion as required in industrial production is not obtained or a very long polymerization time is necessary in order to obtain a high conversion, and therefore, a productivity is very low. Also, when an organic peroxide such as benzoyl peroxide is employed by suspension or bulk polymerization, the copolymer is not obtained in an industrially satisfactory high conversion even at an optimum temperature for the peroxide.
Like this, the .alpha.-methylstyrene/acrylonitrile copolymers prepared by a conventional suspension or bulk polymerization process cannot provide the copolymers capable of industrially utilizing as molding materials, or even if obtained, the productivity is very low.
It is an object of the present invention to provide a process for preparing a .alpha.-methylstyrene/acrylonitrile copolymer having excellent transparency, heat resistance and strength in a high conversion in a short time by a suspension or bulk polymerization technique.